In the conventional fracturing of wells, producing formations, new wells or low producing wells that have been taken out of production, a formation can be fractured to attempt to achieve higher production rates. Proppant and frac fluid are mixed in a blender and then pumped into a well that penetrates an oil or gas bearing formation. High pressure is applied to the well, the formation fractures and proppant carried by the fracturing fluid flows into the fractures. The proppant in the fractures holds the fractures open after pressure is relaxed and production is resumed. Various fluids have been disclosed for use as the fracturing fluid, including various mixtures of hydrocarbons, liquefied petroleum gas, nitrogen, and carbon dioxide.
Proppant addition can be added into a pressurized stream of frac fluid, for example liquefied petroleum gas, directly by having the proppant addition tank itself contained under pressure. Proppant addition systems into LPG, such as those disclosed in WO/2007/098606, often use centrifugal pumps to dynamically seal the proppant from the volatile stream of frac fluid. However, a pressure vessel is still required, as the dynamic seal is only present whilst the centrifugal pump is in operation. Systems have been proposed to avoid the use of a pressure contained proppant tank, for example by sending a stream of proppant blended with frac oils, and a stream of liquefied petroleum gas as (LPG) to separate frac pressure pumps, after which the two streams are combined at pressure and then used to frac a well. This system requires the use and coordination of multiple sets of frac pressure pumps, which are expensive and costly to operate. The outlet pressures from the two sets of frac pressure pumps must be balanced correctly, which makes the pumping difficult to control. It also requires that the mixture of proppant be mixed with substantial amounts of low vapor pressure frac oils, which may seriously reduce the positive effects of the LPG frac fluid, namely easy clean up and recovery from the well.